linux-kernel - Re: [PATCH v2] ftrace: simplify ftrace hash lookup code

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Date:   Tue, 10 Sep 2019 05:28:04 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Changbin Du <changbin.du@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ftrace: simplify ftrace hash lookup code

On Tue, 10 Sep 2019 08:33:23 +0800
Changbin Du <changbin.du@...il.com> wrote:

> > 
> > bool ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip, bool empty_result)
> > {
> > 	if (ftrace_hash_empty(hash))
> > 		return empty_result;
> > 
> > 	return __ftrace_lookup_ip(hash, ip);
> > }
> >  
> We must add another similar function since ftrace_lookup_ip() returns a pointer.
> 
> bool ftrace_contains_ip(struct ftrace_hash *hash, unsigned long ip,
> 			bool empty_result)
> {
> 	if (ftrace_hash_empty(hash))
> 		return empty_result;
> 
> 	return !!__ftrace_lookup_ip(hash, ip);
> }
> 
> But after this, it's a little overkill I think. It is not much simpler than before.
> Do you still want this then?
> 
>

Or...

static struct ftrace_func_entry empty_func_entry;
#define EMPTY_FUNC_ENTRY = &empty_func_entry;

[..]
 * @empty_result: return NULL if false or EMPTY_FUNC_ENTRY on true
[..]
 * @empty_result should be false, unless this is used for testing if the ip
 * exists in the hash, and an empty hash should be considered true.
 * This is useful when the empty hash is considered to contain all addresses.
[..]
struct ftrace_func_entry *
ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip, bool empty_result)
{
	if (ftrace_hash_empty(hash))
		return empty_result ? EMPTY_FUNC_ENTRY : NULL;

	return __ftrace_lookup_ip(hash, ip);
}

But looking at this more, I'm going back to not touching the code in
this location, because __ftrace_lookup_ip() is static, where as
ftrace_lookup_ip() is not, and this is in a very fast path, and I
rather keep it open coded.

Lets just drop the first hunk of your patch. The second hunk is fine.


-- Steve